It is common for a gas turbine engine to include a rotor shroud. The rotor shroud is typically located downstream of a high pressure vane (“HP vane”) in the gas turbine engine, usually with a radially inner surface of the rotor shroud facing the unshrouded tips of the blades of a high pressure turbine (“HP turbine”). The rotor shroud is usually a ring shaped structure (or “annulus”) and is typically formed from a plurality of arcuate segments mounted to a structural casing in the engine.
In use, the rotor shroud typically contains hot combustion gasses produced in a combustor of the gas turbine engine as those hot combustion gases pass through a rotor passage which contains the blades of the HP turbine. Consequently, the rotor shroud is typically subject to high heat loads, particularly at its radially inner surface. Moreover, the passing of the rotor tips typically imposes periodic pressure fluctuations of large amplitude on the radially inner surface of the rotor shroud.
Different cooling configurations have previously been proposed for the rotor shroud. A typical design uses an imperforate casing coated with a thermal barrier coating and an internal cooling circuit. Other designs utilise film cooling holes fed by cooling air, usually bled from a compressor in the gas turbine engine, via plenums within the arcuate segments of the rotor shroud, so as to film cool the radially inner surface of the rotor shroud immediately downstream of the HP vane and through the rotor passage.
Examples of film cooled rotor shrouds are described, for example, in U.S. Pat. No. 7,147,432, US2012/0057961, U.S. Pat. No. 7,296,967, U.S. Pat. No. 6,354,795, U.S. Pat. No. 6,196,792.
An example of a rotor shroud that uses a thermal barrier coating (“TBC”) is described, for example, in U.S. Pat. No. 4,497,610.
An HP vane in a gas turbine engine is typically situated downstream of a combustor in the gas turbine engine. The HP vane is typically subjected to high heat loads due to its proximity to combustion gases. The HP vane is particularly difficult to cool since there is not usually adequate space for an internal cooling circuit to be placed at the tip of the trailing edge of the HP vane. The HP vane is also usually subjected to an unsteady pressure potential field generated by the downstream HP rotor. Typically cooling of the trailing edge of the HP vane is accomplished using a slot in the HP vane which is optimised for aerodynamic design rather than temporal control of the flow rate of cooling air through the slot.
The present inventor has observed that prior art rotor shroud cooling designs are typically subjected to large pressure fluctuations associated with the turbine rotor pressure field (the frequency of such fluctuations typically occur in the range 10-20 kHz, which corresponds to the typical passing frequency of the rotor blade tips). The present inventor has noticed that this unsteady pressure field causes large unsteady variations in ejected temporal coolant mass flow rate from the exit of the cooling holes onto the radially inner surface of the rotor shroud. In the rotor frame of reference, this typically results in a higher than average amount of coolant being ejected onto the rotor shroud surface local to the rotor suction surface and under the rotor tip when the time instantaneous pressure ratio across the holes is high. This flow is typically entrained into the rotor tip leakage vortex and subsequently has a limited cooling effect. Conversely in the region local to the rotor pressure surface less coolant is ejected onto the rotor shroud surface due to the lower instantaneous pressure ratio across the cooling holes. This is the region subjected to the largest heat loads and is therefore the region that it would be most beneficial to cool.
The present inventor has also observed that the unsteady nature of this process can lead to temporal ingestion within the film cooling holes local to the rotor pressure surface, even if the plenum pressures are set to exceed the maximum temporal exit pressure ratio. Without wishing to be bound by theory, the present inventor believes that this ingestion is caused by a sudden rise in film cooling hole exit pressure (resulting from the passing of the rotor tip), which sends a compression wave up the cooling hole, which in turn induces a change in the bulk coolant flow velocity within the hole, which can in some cases cause a bulk flow reversal within the hole leading to ingestion.
The present inventor believes that a similar mechanism exists for trailing edge slots in HP vanes. In this case, the pressure fluctuations are caused by an unsteady pressure potential field generated by the downstream HP rotor.
The present invention has been devised in light of the above considerations.